Here's why:
* Ragdoll coat patterns are complex. They are determined by a combination of genes, and the interaction of these genes creates different color and pattern combinations.
* White is often associated with specific patterns. In ragdolls, a white belly is usually associated with the "mittened" pattern, where white markings extend from the paws to the belly. The "colorpoint" pattern, which typically features a darker body with lighter points, generally doesn't include white on the belly.
* Black is a dominant color. While black is a dominant color in ragdolls, it's unlikely to completely overpower the white markings in a mittened pattern.
However, exceptions can happen. Genetics is complex, and there is always a chance of a unique color and pattern combination. It's possible, but not very likely, that a ragdoll could be born with a white stomach and black face.
If you're looking for a specific color and pattern in a ragdoll, it's best to find a reputable breeder who can show you the lineage of their cats. They can help you understand the possible color and pattern combinations and find a kitten that matches your preferences.